1. Field of the Invention
The present invention relates to a matrix-type display device having a function to correct bus line break troubles.
2. Description of the Prior Art
As image display devices replace CRTs (cathode ray tubes), matrix-type display devices utilizing liquid crystals, EL emitters, plasma beam emitters, or the like are attracting attention. In particular, liquid crystal display devices are widely used in various applications, such as portable TV sets, word processors, and personal computers. It is desired that display devices employed in these applications be more sophiscated and larger in size.
In order to enable minute images to be displayed by employing a matrix-type display device, it is necessary that picture elements constituting a matrix be smaller in size and much greater in number. An increase in the number of picture elements requires an increase in the number of bus lines that function as scan and signal lines. The larger the number of bus lines, the more is line breaking likely to occur. Moreover, as display devices become larger in size, bus lines become larger in length. As such, it is now increasingly becoming difficult to fabricate break-free bus wiring.
There has been developed a matrix-type display device having a function to correct bus-line breaks. A circuit diagram for a display electrode board as used in the display device is shown in FIG. 15A. A large number of source bus lines 1 extending in parallel are arranged in orthogonal relation to a large number of gate bus lines 2 extending in parallel in a direction. Each gate bus line 2 and each source bus line 1 function as a scan line and a signal line, respectively, and intersect each other in a non-conducting state so as to sandwich a gate insulation film 23 (FIG. 15B) therebetween which will be described hereinafter. Each rectangular area defined by adjacent gate bus lines 2 and adjacent source bus lines 1 has a pixel electrode 12. A liquid crystal is sealed between the pixel electrode 12 and the counter electrode 121 to form a liquid crystal cell 16. A thin film transistor (hereinafter to be referred to as "TFT") 11, as a switching element, is connected to each pixel electrode 12. A gate electrode 13 of and a source electrode 14 of each TFT 11 are respectively connected to the gate bus line 2 and to the source bus line 1. The TFT 11 has a drain electrode 15 connected to the pixel electrode 12.
Spare lines 3 are placed along the outer limits of the entire area in which gate bus lines 2 and source bus lines 1 intersect one another. The spare lines 3 comprise two spare lines 3a and 3c parallel to the gate bus lines 2 and two spare lines 3b and 3d parallel to the source bus lines 1. The four spare lines 3a, 3c, 3b, 3d are electrically connected to one another.
FIG. 15B shows a sectional view taken along a gate bus line 2 at point B in FIG. 15A. A base coat film 5 is deposited on the entire surface of a glass substrate 25, and a gate bus line 2 is pattern-formed on the base coat film 5. A gate insulation film 23 is deposited on the entire surface of the gate bus line 2, and the spare line 3b is placed on the gate bus line 2 in an intersecting relation therewith so as to sandwich the gate insulation film 23 therebetween. Further, on the entire surface of the spare line 3b there is formed a protective film 26.
FIG. 15C shows a section taken along a source bus line 1 at point C in FIG. 15A. The spare line 3a is pattern-formed on the above mentioned base coat film 5, and the above mentioned gate insulation film 23 is placed on the entire surface of the spare line 3a. A source bus line 1 is placed on the spare line 3a in an intersecting relation therewith so as to sandwich the gate insulation film 23 therebetween.
In a display electrode board having such a circuit, a breakdown caused to any gate bus line 2 or source bus line 1 is recognized as a line defect on a display screen, and it constitutes a big factor which can decrease the display yield. In this display electrode board, any breakdown caused to the gate bus lines 2 or source bus lines 1 is corrected by using the spare lines 3. For example, as FIG. 15A shows, where a breakdown spot 4 is present on one source bus line 1x, the defective source bus line 1x and the spare bus lines 3a and 3c are electrically connected at points 4a and 4b where both ends of the defective source bus line 1x intersect the spare lines 3a and 3c respectively. This connection is effected by applying energy such as a laser beam to the intersecting points 4a and 4b to thereby destroy the gate insulation film 23. Through such a connection at two points, portions of the defective source bus line 1x at both sides of the breakdown spot 4 are electrically connected via the spare lines 3a, 3c, and 3d.
However, such a way of correction gives rise to the following problems. First, the problem of load capacity can be pointed out. The load capacity C.sub.2 of the source bus line 1x to which the spare lines 3 are connected (hereinafter to be referred to as "corrected source bus line") equals a value corresponding to the normal load bus capacity C.sub.0 of the source bus line 1 plus the load capacity C.sub.1 of the spare lines 3. That is, C.sub.2 =C.sub.0 +C.sub.1. The length of the spare lines 3 is about equal to the total length of two gate bus lines 2 and two source bus lines 1 and, therefore, the value of the load capacity C.sub.1 is considerably larger than that of C.sub.0. The capability of a driver IC which supplies signals to pixel electrodes 12 through the source bus lines 1 is designed on the basis of the normal load capacity C.sub.0 of the source bus lines 1 and, therefore, it is insufficient for driving the pixel electrodes 12 through the corrected source bus line 1x which has such a large load capacity C.sub.2. Therefore, picture elements formed by the pixel electrodes 12 connected to the corrected source bus line will display themselves in a condition different from those formed by pixel electrodes 12 connected to other normal source bus lines 1. As such, even though correction is made by means of the spare lines 3 in the above mentioned manner, it cannot be said that the breakdown spot 4 has been completely corrected.
Secondly, the problem of signal delay is pointed out. Assuming that signals are supplied from one end nearer to the intersecting point 4b on the corrected source bus line 1x, signals are supplied through the spare lines 3 to the portion between the breakdown spot 4 of the corrected source bus line 1x and the intersecting point 4a. Each signal is transmitted over a longer distance than other normal source bus lines 1 until it reaches the intersecting point 4a. In this way, the total length of the corrected source bus line is longer than the normal source bus lines 1, which involves greater electrical resistance, resulting in signal delay. Picture elements formed by the pixel electrodes 12 suffering from considerable delay in signal input will display themselves in a condition different from those formed by pixel electrodes 12 connected to other normal source bus lines 1. As such, even though correction is made by means of the spare lines 3 in the above mentioned manner, it cannot be said that the breakdown spot 4 has completely been corrected.
Thirdly, the problem of signal noise is pointed out. Since the spare lines 3 intersect all of the gate bus lines 2 and source bus lines 1, the corrected source bus line 1x connected to the spare lines 3 are subject to the influence of signals on these many bus lines which intersect the spare lines 3. The effect of these signals extend to the spare lines 3, with a result that noise is produced. This noise is considerably larger than any possible noise from normal source bus lines 1 having no portion intersecting other source bus lines 1. Therefore, picture elements formed by the pixel electrodes 12 connected to the corrected source bus line 1x may display themselves in a condition different from those formed by other normal pixel electrodes 12. In such a case, if correction is made by means of the spare lines 3 in such a manner as mentioned above, the breakdown spot 4 cannot completely be corrected.
Any or all of the foregoing problems will occur noticeably as the display device becomes larger in size. Since, as stated above, line breaks are more likely to occur with an increase in the size of the display device, the necessity of solving the foregoing problems becomes greater as the size the display device becomes larger.
Fourthly, in display devices having the above-mentioned spare lines 3, the trouble of defective isolation is likely to occur between the spare lines 3 and the gate bus lines 2 or source bus lines 1. If a spare line 3 on which such a defective isolation problem is present is used to correct the breakdown spot 4, such correction may result in a new defect being caused to other part or parts. For example, at the intersecting point 4c shown in FIG. 15A, let it be assumed that a defective insulation problem is present between the source bus line 1y and the spare line 3a. Where such a defect is present, if the defective source bus 1x on one hand and the spare lines 3a and 3c on the other are interconnected at intersecting points 4a and 4b as earlier mentioned, the source bus lines 1x and 1y are electrically connected, with a result that a line defect is caused to the picture elements connected to the source bus line 1y. In display devices in which such spare lines 3 are provided, possible troubles of defective isolation between the spare lines and the bus lines pose a serious problem.